Do you mean like the Prost wing where a wing element goes across the top of the wing and use that as the mount ? If so I have though the same thing.

I know Prost still used the drop plates going all the way down to the lower element, they could equally could have stopped at the upper element. The set up could work as an option for higher downforce wings, the element would need to be quite strong in Beam stiffness, this may make the element have a very large cross section. With a lowish nose the mount could be horizontal and still meet the nose cone without going outside of the regulation dimensions.

The lower picture is with conventional vertical wing mounts (blue), the upper one shows the horizontal element\mount (blue)



The gain for this set up may be small, the normal vertical mounts are strong and light, most of a wings work is done on its under surfaces so there's a very small aero gain, the horizontal element may be heavier and is placed higher up going against a low CofG.